1. Field of the Invention
The present invention relates to improvements in semiconductor switch devices, and more particularly to a method for supplying a current to the control electrode of a semiconductor switch, e.g., a transistor switch.
2. Description of the Prior Art
In semiconductor switches, there has been a growing interest in the development of simplified means for supplying and controlling the control electrode current. In practice, such control current, if considerably large, can hardly be isolated and on-off controlled. To do this, prior art techniques have had to use a control electrode power source capable of isolated power supply for each switch unit, which has necessitated transformers and rectifiers of large capacity. When a large control electrode current is necessary to cope with a maximum main current, the power source must be large enough to deal with such current. One prior art approach to this problem is the method relevant to transistor inverters in which an AC current transformer is used for operation responding to a main current flow in the semiconductor switch, and the transformer output current is supplied to the control electrode of the semiconductor switch. In this method, however, the AC current transformer is inoperable for main current containing a DC component. Hence this method is virtually impractical for chopper control devices used for turn-on time controls. In applications to relatively low frequency inverters such as commercial frequency inverters and variable frequency inverters, the prior art approach is not practical, particularly in view of the fact that it requires a current transformer of large capacity.